1. Field of the Invention
The present invention relates to a fluorescent lamp that includes an end cap formed of synthetic resin, a manufacturing method for such a fluorescent lamp, and to a lighting apparatus that uses the same.
2. Prior Art
A known problem for fluorescent lamps is that the temperature of the electrode seals at the ends of a fluorescent tube rises sharply as the fluorescent tube approaches the end of its operating life.
The improvements in efficiency of fluorescent lamps and the concurrent reductions in their size and weight have led to an increase in the use of fluorescent lamp apparatuses. Such apparatuses use high-frequency inverter lighting circuits to light the fluorescent tubes. However, a particular problem has been observed when a fluorescent tube used in such an apparatus reaches the end of its operating life. When the emissive material that is originally present within the electrode filament has been dispersed, the cathode drop voltage increases and so prevents the fluorescent lamp from illuminating. However, the high-frequency inverter lighting circuit will continue to supply a preheating voltage to the electrode filament, so that electrode filament will remain in its preheated state. As a result, arc discharge occurs between the lead wires inside the electrode which increases the temperature of the electrode seal.
It is believed that the above problem is caused by the high current-feed capacity of the high-frequency inverter lighting circuit.
The above problem is especially prevalent in compact single-ended fluorescent lamps where the fluorescent tubes have a relative small diameter and two electrode seals are enclosed within a single end cap formed of synthetic resin. In conventional compact single-ended fluorescent lamps, the electrode seals that act as the problematic heat sources are located alongside one another, so that the aforementioned rise in temperature is especially marked. Heat dissipation is also poor, since the synthetic resin end cap encloses the two electrode seals. These factors result in an excessive rise in temperature which in extreme cases can lead to deformity in the end cap.
Japanese Laid-Open Patent Application H02-192650 discloses a technology that can prevent such rises in temperature at the ends of a fluorescent lamp. A thermal fuse is provided near the end of the fluorescent tube, so that when the temperature at the end of the fluorescent tube rises at the end of the operating life of the fluorescent tube, the thermal fuse will melt, thereby breaking the lighting circuit.
This technology has the thermal fuse located on the outside of the fluorescent lamp. This results in the thermal fuse being visible, which spoils the external appearance of such lamps. When a fluorescent lamp is formed by connecting four or more fluorescent tubes to a single end cap, it would be possible to keep the thermal fuses hidden from view by placing them in a space on the back side of the end cap. However, no such space is conventionally available on an end cap, so that it has only been possible to provide the thermal fuses on the outside of the cap where they will be clearly visible.